Wednesday, August 31, 2016

The Trouble with the Plankton

Earlier this summer, I went to a Gordon Research Conference
on Ocean Global Change
Biology at the invitation of Sinead Collins. I don’t typically work on
oceans, and so the fit might not see obvious, but the relevant part was that
the field has taken on a distinctly evolutionary flavor. It turns out that many
ocean biologists are now focusing on adaptive responses of marine organism to
climate change, especially ocean acidification. It was a wonder to sit through
talk after talk of studies assessing the potential for (usually) plankton to
adapt to either increased acidity or warmer water. Even the talks that didn’t
focus on evolution almost always referred to it in an informed and considered
manner. I had previously been to a similar conference 8 or so years earlier,
and that time I saw only the barest hint of evolution – so this was an exciting
change. Yet it isn’t in my nature to be complementary without qualification (or
critical without qualification) – and the same will apply here.

I couldn't decide my favorite photo so here are Google's favorites.

In 1961, G. Evelyn Hutchinson wrote a paper titled The Paradox
of the Plankton, in which he discussed the apparent paradox that so many
species of plankton coexist even though they compete for similar nutrients –
ostensibly in contradiction to the principle of competitive exclusion. I wish
to here introduce – by way of verbal analogy – The Trouble with the Plankton,
which is somewhat related to the Paradox of the Plankton in its emphasis on
variation.

Stated simply, I suggest that Ocean Global Change biologists
should stop worry about whether or not plankton will evolve in response to
climate change – they will! Unlike many other organisms, evolution is not normally
going to be a problem for phytoplankton (or even zooplankton) – for four main
reasons.

I couldn't decide my favorite photo so here are Google's favorites.

1. Most species of plankton are extremely abundant,
which means that standing genetic variation will be huge, as will be mutational
inputs. In short, genetic variation – the raw material for evolution – should be
massive for essentially all marine plankton.

2. Most species (and indeed many populations and
even individuals) of plankton experience dramatic fluctuations in environmental
conditions across space (vertical and horizontal) and time (hourly, daily,
seasonally). This past variation in environmental conditions means that past
selection will have tested (and sometimes favored) adaptive genetic variants
for a wide variety of conditions – again maintaining high genetic variation in
adaptively-relevant variants.

3. The rate of abiotic environmental change in the
ocean is very modest not only in relation to the above-noted past and present
spatiotemporal variation in selection but also in relation to the generation
time of phytoplankton (and zooplankton). As a result, the per-generation shift
in the environment owing to climate change will be tiny in relation to the
potential evolutionary speed of plankton.

4. Many plankton show adaptive plasticity in
response to different abiotic conditions, including acidity and temperature.
This plasticity should buffer the immediate negative effects of environmental change
and thus allow further time for evolution.

Fitting these expectations, every study at the conference
showed strong evolution in response to dramatically altered environments (often
much more so than projections for climate change), despite often extremely limited
starting genetic variation. Many studies of freshwater plankton have similarly
shown that evolution in even small experimental populations can accomplish – in
only a single summer – full adaptation to environmental changes projected to
take place over decades. And “resurrection” studies that bring past zooplankton
to life also show rapid responses to all sorts of environmental changes. So I
suggest that we don’t need more studies asking “can plankton adapt to climate
change” – they can – simple as that.

However, I do think that further evolutionary studies are
critical for Ocean Global Change Biology – I merely suggest that their focus
should be a bit different.

1. Studies could profitably ask “what are the
consequences of the evolution of plankton for communities and ecosystems.” I
image that the evolution of plankton in response to climate change could
dramatically alter their relationship with other species in the community. Some
of those species, especially those with longer generation times, such as
planktivorous fish, might have trouble responding adaptively. Thus, it would be
fascinating to take those experimentally evolved lines of plankton and see how
they interact with other key species in the community.

Here you can find more arguments for considering the ecological effects of evolution.

2. Although most (maybe all) plankton will have no
trouble adapting to abiotic changes associated with climate change, they might
have trouble adapting to some correlated biotic changes. For instance,
planktivorous fish might dramatically change in abundance with climate change,
which might then impact plankton populations in ways that are strongly modified
by evolution.

Of course, the general statements above are not intended to
imply that all marine invertebrates will easily adapt to climate change. Corals
for instance seem to be near their physiological (and evolutionary) limits
already and might have no suitable genetic variation to respond to selection.
Of course, changing their symbionts might be another way to adapt – although that
too will have limits. Also, species in already extreme conditions (e.g., the hottest
or most acidic water) might not be able to persist locally as those conditions
change. Indeed, acid rain caused the extirpation of many (but not all) plankton
species – and very warm (or cold) temperatures could do the same.

Regardless of whether or not I am correct that
plankton will have little trouble adapting, I do think evolutionary studies are
extremely informative. I can’t wait to be invited back for the next Gordon
Research Conference – or perhaps I won’t be given this post.